For over 30 years, companies have searched for ways to construct a thin, low-power version of the conventional cathode ray tube (CRT). These efforts have led to a number of flat panel display technologies. None, including liquid crystal displays (LCDs) have met all of the needs for improved power, brightness, efficiency, video response, viewing angle, operating temperature, packaging, full color gamut, ruggedness, and scaleability.
Among the obstacles encountered in fabricating thin cathode ray displays is the deposition of a lacquer layer on the faceplate of the display prior to adding an aluminum layer. The aluminum layer is used to act as a mirror behind each sub-pixel in the display faceplate to reflect the light photons back toward the phosphors of the display screen to create a brighter image. Surface irregularities in the aluminum layer scatter these photons and reduce the efficiency of the aluminum layer in reflecting light to the phosphors, thus degrading the brightness of the display. The lacquer layer provides a supporting structure when the aluminum layer is deposited so that the aluminum layer is deposited upon an even surface and will reflect light evenly toward the phosphors.
One method of depositing the lacquer layer is known as a “float lacquer” process. FIGS. 1A-C are cross section views showing the steps in a prior art float lacquer process 100. In FIG. 1A, a faceplate 101 is submerged in a solvent 102. In FIG. 1B, a thin layer of lacquer 103 is deposited or floated on top of solvent 102. The solvent is then drained from the tank and, as the solvent level subsides, lacquer layer 103 is deposited upon faceplate 101. In FIG. 1C, the level of solvent 102 in the sub-pixels 104 of faceplate 101 is then further reduced by evaporation and an aluminum layer is deposited directly on top of lacquer layer 103. If the aluminum layer were to be deposited directly upon the phosphor rocks within sub-pixels 104, it would conform to the surface of the phosphor rocks and have a very irregular surface which would reflect light back to the phosphor rocks unevenly. During a subsequent baking operation, the remnants of lacquer layer 103 are removed as they can cause phosphor degradation if it remains.
The float lacquer process, however, is time consuming and is vulnerable to operator error. The amount of time it takes to set up the float tank and allow the solvent to become still enough to deposit lacquer layer 103 means the process is not well suited to larger scale manufacturing processes. Additionally, there can be variations in lacquer layer 103 as large as 30% using the float lacquer process, resulting in an irregular aluminum surface. This causes a nonuniform screen appearance and degrades the efficiency and brightness of the display.
The structure of thin CRTs limits the choice of lacquers in a float lacquer process to soft materials with very high elongation. High elongation is necessary to obtain a scaffold for the reflective aluminum to be applied without “tenting” over the rows and columns between pixels. Tenting can be caused by an excessive amount of lacquer on the faceplate which makes the surface of the aluminum balloon and rupture when the lacquer and remaining solvent is baked out. Tenting can be detrimental, not only to the faceplate, but also during final assembly when support structures, inserted to provide greater structural integrity, can cause the aluminum layer to break which leads to electrical arcing in the finished display assembly. Tenting causes non-uniform screen appearance and reduced efficiency and brightness.
Materials with high elongation are also soft materials, which means that the lacquer layer will be very conformal around the phosphor in the sub-pixels. In FIG. 2, a highly conformal lacquer layer 201 has been deposited upon a layer of phosphor rocks 202 contained in a sub-pixel 203. An aluminum layer deposited upon this lacquer layer will take on the shape of the conformal lacquer layer during the subsequent baking step to remove the lacquer layer and any remaining solvents. This causes the aluminum to also take on an irregular shape which reduces the reflectivity of the aluminum layer and can cause a grainy appearance in the display due to bad uniformity. To smooth the aluminum, a thicker lacquer layer (>1μ in thickness) is usually deposited on a regular CRT. Due to the lower voltages used in a thin CRT, a thinner layer of aluminum is necessary to prevent excess electron energy loss. However, this thin aluminum layer is susceptible to blistering and breakage during the bake out if the lacquer layer is greater than 1μ in thickness. In summary, using a thin lacquer layer creates an excessively conformal aluminum layer and using a thicker lacquer layer leads to tenting and rupturing of the aluminum layer.
Accordingly, the need exists for a method of producing a non-conformal lacquer layer for a display device which will result in a smooth, highly reflective aluminum layer that is electrically and mechanically robust. It is also desirable that this method, while meeting the above stated needs, should be applicable to large scale manufacturing processes.